A fuel injector 1 will be described by way of background with reference to FIG. 1. The injector 1 comprises a nozzle body 3, an injector nozzle 5 and a movably mounted injector needle 7. The injector nozzle 5 comprises a plurality of nozzle holes 9 which can be selectively opened and closed by the injector needle 7 to inject fuel into a combustion chamber (not shown). Specifically, the injector needle 7 has a lower valve 11 for cooperating with a lower valve seat 13 formed in the injector nozzle 5. A spring 15 is provided in a spring chamber 17 for biasing the injector needle 7 in a downwards direction to seat the lower valve 11 in the lower valve seat 13, thereby closing the nozzle holes 9.
An upper end of the injector needle 7 extends into a control chamber 19 formed in a piston guide 20. The control chamber 19 is in fluid communication with the spring chamber 17 via an inlet orifice 21. A drain pathway 23, having a restricted drain orifice 25, forms a fluid pathway from the control chamber 19 to a low pressure fuel return line (not shown). The injector needle 7 has an upper valve 29 for cooperating with an upper valve seat 31 formed in the nozzle body 3 to seal the control chamber 19. A 3-way control valve (not shown) is provided for selectively opening and closing the drain pathway 23 to control the fuel pressure within the control chamber 19. The 3-way valve is actuated by an electro-mechanical solenoid (not shown).
A fuel supply line 33 supplies high pressure fuel from a fuel pump (not shown) to the injector nozzle 5 and the spring chamber 17. The control chamber 19 is selectively in fluid communication with the fuel supply line 33 via the inlet orifice 21. When the injector needle 7 is lifted, the upper valve 29 locates in the upper valve seat 31 and the control chamber 19 is isolated from the inlet orifice 21.
When the 3-way control valve is closed, there is no fluid communication between the control chamber 19 and the low pressure fuel return line. Accordingly, the fuel pressure in the injector nozzle 5 and the spring chamber 17 equalises and the spring 15 biases the injector needle 7 to a closed position in which the lower valve 11 is seated in the lower valve seat 13 and the nozzle holes 9 are closed, as shown in FIG. 1.
Conversely, when the 3-way control valve is opened, a path is formed which places the control chamber 19 in fluid communication with the low pressure fuel return line 27 and the fuel pressure in the control chamber 19 is reduced. Accordingly, the fuel pressure in the injector nozzle 5 is higher than the fuel pressure in the control chamber 19 and a pressure force applied to the injector needle 7 overcomes the bias of the spring 15. The injector needle 7 is displaced upwardly unseating the lower valve 11 from the lower valve seat 13. The nozzle holes 9 are thereby opened and fuel is injected from the injector nozzle 5 into the combustion chamber. The upwards displacement of the injector needle 7 causes the upper valve 29 to be seated in the upper valve seat 31 thereby closing the drain pathway 23 and inhibiting the flow of fuel to the low pressure return line.
The injector needle 7 can move between two steady state positions (fully open or fully closed). The opening and closing velocity of the injector needle 7 is controlled by the balance of pressures on the injector needle 7 as well as the biasing force applied by the spring 15. The opening and closing velocities are determined by the balance of pressures which, in part, relate to the component geometry. The maximum lift of the injector needle 7 is determined by component geometry. The sizing of the inlet orifice 21 and the outlet orifice 25 provide the main control for the speed that the injector needle 7 can move. As the 3-way control valve is opened, fuel escapes but is re-supplied via the inlet orifice 21. If the inlet orifice 21 is larger in comparison to the outlet orifice 25, damping of the lift of the injector needle 7 is increased. Conversely, if the inlet orifice 21 is smaller in comparison to the outlet orifice 25, the speed at which the injector needle 7 lifts is increased.
The fuel injector 1 can be used to inject fuel having a rate shape as illustrated in FIG. 2. The rate shape can be affected by rail pressure, but there is no ability to fundamentally adjust its profile (for example, the initial injection rate or closing rate) during operation.
An ‘intensifier type’ system can be used to generate injection rate flexibility within a common rail system, but still presents some limits on what rate shapes can be achieved. In addition intensifier systems generally have, by design, inherent hydraulic inefficiencies due to the way that the intensifier piston is hydraulically driven.
The present invention, at least in preferred embodiments, sets out to provide an improved fuel injector.